A multilayer disk structure based on a CoCrPtO magnetic layer is one of the material choices for fabrication of perpendicular magnetic recording media. In these devices, nano-sized (typically ~10 nm in diameter), magnetically hard cobalt-rich alloy grains are utilized for information recording, for which a smaller grain size and a narrower grain size distribution are of technical importance. It has been suggested that the thickness of the Ru-based interlayer (typically ~25 nm thick) deposited prior to the magnetic layer is influential in determining the average grain size of the magnetic layer. High resolution TEM images were obtained for several CoCrPtO perpendicular magnetic recording medium samples with different interlayer thicknesses. The average grain sizes were measured directly on the TEM images and their distributions were analyzed. The results confirmed the existence of a relationship between the interlayer thickness and the grain size of the magnetic layer. The grains grow in size up to a certain maximum diameter, at which they undergo a division into multiple subgrains reducing the average grain size. This mechanism clearly limits the upper bound of the grain size distribution. Bright field and high resolution TEM images of the nanostructured samples along with grain size distributions obtained will be presented.